Speech constitutes children's primary linguistic data, occupying a fundamental causal role in acquisition: children to whom English speech is addressed learn English, children to whom Mandarin speech is addressed learn Mandarin, and so forth. Child-directed speech is a unique register that, among other things, sounds different than other types of speech. Global acoustic characteristics of child-directed speech have been investigated in a number of studies dating from the middle 1970's. However, more fine-grained acoustic characteristics of chid-directed speech, which are critical for determining whether and how grammatical information in encoded, remain largely unexplored. The hypothesis motivating the proposed research is that input speech may encode information on abstract aspects of grammar, providing bases for an initial rough categorization of words into form classes and for parsing sentences into major constituent phrases. To the extent that this account is correct, existing theories assuming that input supports only an impoverished representation and attributing to the child a complex, specifically linguistic learning apparatus will be challenged. This hypothesis may also have ramifications for treatment of language deficits, suggesting that such deficits may arise less from central difficulties in grammatical analyses and more from peripheral difficulties in organizing the information available in input. The proposed studies are designed to begin to explore several fundamental issues. How does input support segmentation of continuous speech into words? Does input speech provide perceptual bases for initial categorization of words into form classes? What aspects of input speech foster children's use of particular words or morphemes? Does input speech provide reliable cues to sentence phrase structure? Taken together, these studies will provide basic descriptive evidence on the acoustic nature on input speech and will provide foundations for further hypothesis-driven investigations of language input.